I'm checking which tags have tag wiki excerpts in my code. It looks like /tags/{tags}/wikis is the right API since the tag info API doesn't even give a boolean for tag wiki excerpt presence.

But there's a bug where the tag wiki excerpts are not returned for the "standard meta tags": support, discussion, featured, bug. (I'm not sure if others are also affected.) These seem to be the tags with either/both of the is_required and is_moderator_only flags.